1. Field of the Invention
The present invention relates to a device for controlling a lock-up clutch in an automatic transmission with a lock-up clutch equipped fluid torque converter and, more particularly, to a device for controlling the lock-up clutch by utilizing the hydraulic pressure supplied to the hydraulic servo means in the transmission.
2. Description of the Prior Art
In automatic transmissions equipped with torque converters having a lock-up clutch, the lock-up clutch is engaged at two different speed ranges and temporarily disengaged when the transmission is changed over between the two speed ranges in order to improve fuel consumption and alleviate shocks caused by the gear-shifting. In U.S. Pat. No. 4,095,486, it has been proposed to properly choose two pressure values based on transitional variation of the oil pressure supplied to the hydraulic servo means incorporated in the transmission so as to obtain the timing for disengaging the lock-up clutch temporarily. However, this U.S. Invention has a disadvantage that the valve structure is complicated.